Last season, the Celtics ranked 4th in the league in defensive rating. So far this season, they rank 29th (per basketballreference.com). That’s bad. Like, really, really bad. Last night they gave up 128 points to the Cavaliers. The Cavs only averaged 104 points against last year's Celtics. So what’s going on? Why are the Celtics so bad on defense this season?

Defense was the Celtics calling card last season. Jae Crowder and Avery Bradley were praised as two of the best perimeter defenders in the NBA, it even got Avery Bradley and All-Defensive team selection. But those accolades have done nothing for the Celtics this season, as they come out of last night ranked in the bottom 5 of opposing points per game, and leave all of us dumbfounded, questioning what happened to the the team we thought we had?

Why are the Celtics so bad on defense this season? Here’s why:

1. Offensive Rebounding

The rebounding has been bad in general for the Celtics. The Celtics replaced Jared Sullinger with Al Horford in the offseason, and while it may have been an upgrade everywhere else, it was a downgrade on the glass. Sullinger averaged a staggering 4.5 more rebounds per 36 minutes than Al Horford (per basketballreference.com). The Celtics ranked 24th in rebounding before last night, and it probably got worse.

​The loss of Sullinger has shown early on this season, and the Celtics don’t seem to have an answer coming anytime soon. That deficiency was in the spotlight Thursday night, as Tristan Thompson pulled down 10 rebounds… in the 1st quarter. Stevens can’t exactly coach his way out of bad rebounding. Jordan Mickey got a lot of minutes last night, but it’s asking a lot of a 2nd year, 2nd round pick, to fill the biggest hole on your team. Especially when Mickey will likely go back to Maine once Horford and Olynyk are back. Al Horford has steadily declined as a rebounder every year he’s been in the NBA since his rookie season. Unless he can find a way to drastically improve last in his career, the Celtics rebounding will be their kryptonite all season long.

2. Lazy Rotations & Lazy Transition Defense

It’s hard to know why, but so far the Celtics have been lazy on defense. They’ve been getting killed this season when they’ve been put in the pick and roll. The man defending the corner comes over to help, leaving his man wide open for a 3. Last season, the defense was close to perfect on rotating on these plays. This season it is leading to open looks for opponents. Here’s just one example from the 2nd quarter of last night’s game:

Isaiah has no reason to be rotating there, it’s a complete lack of awareness on the part of the Celtics. The Tristan Thompson poster was another good example of how bad they’ve been against pick & roll plays. It was all but a free run to the hoop after Tyler Zeller placed himself in perfect position to get posterized. The Celtics can fix this by just getting their defenders to cut off passing lanes out of the pick & roll, but they haven’t shown the ability or willingness to do that so far this season.

The Celtics also have been lazy in transition. Last night there was multiple occasions where Celtics defenders were out to lunch getting back, leaving people completely unguarded. The frustrating part is that this was even happening after Celtics makes. Look at how easily Kevin Love gets to the hoop on this play:

The Celtics need to be more disciplined on defense if they’re going to be anywhere near the team they were last year. Teams will continue to attack Isaiah on defense, so excellent help defense and rotating is vital to keep the defense efficient with Isaiah on the floor.

3. Size

When the Celtics go small, they might be the smallest team in the league. With Isaiah Thomas (5’9”), Avery Bradley (6’2”), and Marcus Smart (6’4”) all on the court at the same time, teams will be able to create mismatches. With or without Jae Crowder, the Celtics need to run less of this lineup. LeBron James shot over Marcus Smart with ease last night when he got that matchup.

The Celtics appear unconcerned about giving up open looks to them, even though they are still capable 3 point shooters. On one particular play last night, Jaylen Brown sagged off so much, he got called for a defensive 3 second violation. Last season, the Celtics ranked top 5 in opposing 3 point percentage. This season, they rank 24th.

The lack of respect the Celtics are showing to Wade, James, Butler, Shumpert, etc. is perhaps the most fixable problem the Celtics have. They just need to stop treating players that can shoot like Deandre Jordan. In the first game against Chicago, Dwyane Wade hit a 3 as the shot clock was winding down from about 3 or 4 feet off the 3 point line. I get if Avery Bradley isn’t scared of Wade’s jump shot, no one really thought he could shoot like this coming into the season, but there's 3 seconds left on the shot clock! All he can do is shoot! And Avery Bradley had his hands down by his side, refusing to even contest Wade’s 3!

The same was true all game for Wade and Butler, and the Celtics lost because of a lack of respect for 2 NBA All-Stars’ ability to shoot. I’m not saying to treat them like Ray Allen, but you still have to GUARD THEM! And the Celtics seem to think they don’t have to in their first 5 games.

The Celtics on paper should have a good defense. Jaylen Brown, Jae Crowder, Avery Bradley, and Marcus Smart should all be elite defenders, and Amir Johnson and Horford should be positives for the defensive side of the ball as well. If the Celtics are going to win a lot of games this season, they need defense. It was their calling card last season, and without it, the Celtics aren’t going to be as good as they should be.